Types of conventional switching mode power supplies include a boost converter topology and a flyback converter topology. A conventional boost converter topology is illustrated in FIG. 1. A boost converter is a power converter which provides an output voltage that is greater than its input voltage (i.e. VL>Vin). The boost converter depicted in FIG. 1 includes an energy storage element, such as the inductor L1, a switching element S1, a flyback diode D1 and a capacitor C1. The output may also be filtered to improve performance.
In operation, the boost converter depicted in FIG. 1 operates in two states, when the switch S1 is on (closed) and when the switch S1 is off (open). When the switch S1 is closed, increased current flows through the inductor L1. When the switch S1 opens, the only path to the increased current is through the diode D1 to the capacitor C1 and the load. This release of energy stored in the inductor L1 results in an increased output voltage. In the idealized circuit, the ratio of the output voltage to the input voltage is 1/(1−D) where D is the duty cycle (0≦D<1) of the switching of switch S1. Thus, the output voltage of the boost converter will be greater than the input voltage. Additional details on the operation of a boost converter topology may be found at en.wikipedia.org/wiki/Boost_converter.
A conventional flyback converter topology is illustrated in FIG. 2. A flyback converter uses a transformer to isolate the input from the output. The voltage Vin is imposed on the output of the transformer T1 according to the turns ratio. When the switch S2 is closed, current flows through the primary side of the transformer T1 but the diode D2 blocks current from flowing through the secondary winding of the transformer T1, increasing the magnetic flux in the transformer T1. Current to the load is supplied by the capacitor C1. When the switch S1 is open, the energy stored in the transformer T1 is supplied through the diode D2 to the capacitor C1 and the load. Additional details on the operation of a flyback converter topology may be found at en.wikipedia.org/wiki/Flyback_converter. An example of a flyback power converter driving strings of light emitting diodes (LEDs) is illustrated in U.S. Pat. No. 6,285,139 to Ghanem.
While boost converter power supplies and flyback converter power supplies could be used to drive a solid state light emitting device as the load, where multiple solid state light emitting devices of different types or different string lengths are employed, such power supplies are not capable of applying different voltages to different types of solid state light emitting devices and/or different string lengths. Where different types of solid state light emitting devices and/or different string lengths are employed, supplying different voltages would provide improved efficiency. See e.g., U.S. Pat. No. 5,812,105 to Van de Ven.
Another difficulty with driving light emitting devices, such as light emitting diodes, relates to current control of the devices. Conventional LED driver circuits have used current regulating bipolar transistors between the strings of LEDs and ground to control current through a string of LEDs. See e.g., U.S. Pat. No. 6,161,910 to Reisenauer et al. Conventional LED driver circuits have also used metal-oxide-semiconductor field effect transistors (MOSFETs) between the strings of LEDs and ground in combination with a pulse width modulated control signal to the MOSFETs to control current through a string of LEDs. See e.g., U.S. Pat. No. 6,362,578 to Swanson et al.